What was found during temple repairs in Josiah's reign?

Answer: The Book of the Law

Hilkiah the high priest found the Book of the Law in the temple.

“Then Hilkiah the high priest said to Shaphan the scribe, “I have found the Book of the Law in the house of the LORD!” And he gave it to Shaphan, who read it.”— 2 Kings 22:8 (BSB)